You installed the valve upside down, you used PEX or some other plastic for the drop down to the tub spout, or the valve needs a "twin ell" and you went directly off the top of the valve, take your pick, but it should be one of these three, and most likely the last one from your description.

And chuck the galvanized while you are at it and go with brass. Unless you like a little rust first thing in the morning when you open the tub valve.
The drop to the tub spout can be full size copper or nipples.

NO CPVC, NO PEX

Grohe rough for a slip-on spout

Hansgrohe rough with a 3/4" drop ear that gets replaced with a 3/4" brass nipple.
The hammer arrestors add nothing of value here. The valve is too slow closing for them to make a difference.

Attached Files:

The top opening on a valve is NOT always for a shower riser, (except for a shower only installation), unless the valve has a "restrictor" in the opening to prevent water from coming out of the head when the tub is filling. I do not believe ANY Grohe thermostatic valve has the restrictor.